1. Field of the Invention
This invention relates to carbon steels for machine structural use, and more particularly to a carbon steel for machine structural use having low deformation resistance in cold forging and having excellent induction hardenability.
2. Related Art Statement
The cold forging is a plastic working method applied over a wide range of from bolt and nut to large-size automobile parts because it has advantages that the finish dimensional accuracy and the yield of material to be forged are excellent and the number of finish cutting steps after the forging becomes less.
Heretofore, steel materials containing less than 0.40% by weight (hereinafter reported simply as %) of C have frequently been used in cold forging. This is because steel materials containing not less than 0.40% of C were high in the deformation resistance during cold forging and were deficient in deformability durable to severe working.
However, has cold forging has recently been applied to steel materials containing not less than 0.40% of C in response to demand for increasing the strength of mechanical parts, particularly surface hardness after quench tempering. In case of using such steel materials, since the increase of deformation resistance is not avoided as previously mentioned, not only the life of the working tool considerably lowers, but also the deformation loading is higher than the capacity of the forging machine and it is required to replace the forging machine with a larger size forging machine.
On the other hand, the reduction of deformation resistance can usually be attained by decreasing the amounts of alloying elements to be added, but the decrease of amounts of alloying elements inversely brings about the degradation of hardenability, so that it was necessary to sacrifice either one of such conflicting properties.
In this connection, various countermeasures for solving the above problems have been proposed from time.
For instance, Cr added steel and Cr-B added steel for reducing deformation resistance without damaging hardenability are disclosed in the Report of Plastic Working Spring Meeting in the year of 1987 (1987. 5. 15.about.17, pp 301-302). Since these steels contain not less than 0.41% of Cr, however, the deformation resistance is still high as mentioned later.
Furthermore, the reduction of deformation resistance and the increase of deformability have been attempted by restricting the amounts of Si, Mn, Cr or further S, P, N, O in Japanese Patent Laid open No. 59-159771 and No. 61-113744. In these techniques, however, the hardenability, particularly induction hardenability, was is still poor, though a reduction of deformation resistance was attained.